Of the seven Democrats seeking their party’s backing to take on Sen. Susan Collins in Maine next year, not one has ever won an election. To anything.

And yet, somehow, a whole lot of Maine Democrats have convinced themselves that because party officials in Washington want Gov. Janet Mills to challenge Collins next year, Mills’ candidacy must be a bad idea.

After all, they point out, when meddling operatives in the nation’s capital pushed Sara Gideon for the task last time around, they got it wrong.

“If we keep running the same kind of playbooks, if we keep having D.C. choose candidates for Maine that don’t always represent the entire electorate of the state, then we run the risk of having another 2020,” oyster farmer and senatorial hopeful Graham Platner told CNN.

The highest public office held by Platner, who has already rapidly gained the support of his party’s progressives, is chair of the Sullivan Planning Board.

I know, I know —  the polls show Collins getting trounced by anyone who can string a sentence together. The New York Yankees probably have a higher approval rating in Maine than Collins does right now.

But the fact remains: Since she won her Senate seat almost 30 years ago, Collins has wiped the floor with every challenger she’s had.

Collins may have reason to be concerned about next year’s race — seriously alarmed, even   — but she is, to a degree largely unrecognized in her home state, a master politician. Knocking her out of office will be very, very hard.

Call me skeptical, but I don’t think that turning to outsiders with little name recognition, no record of political success and no experience in elected office is the best way to oust Collins.

Part of Gideon’s problem was that she’d never sought election to anything except a state House seat in Freeport. That’s better than nothing, but it’s not a resume solid enough for the — considerable — task of defeating Collins.

That brings us back to Gov. Mills, who is too moderate and too old to fit the template for the party’s perfect choice. But she’s won two straight statewide elections relatively easily, one of them against former Gov. Paul LePage, who twice before had persuaded Mainers to install him in the Blaine House.

In short, Mills has something none of the existing Democratic Senate candidates have: proof she can win.

What’s more, Mills has also demonstrated that she’s willing to go toe-to-toe with President Donald Trump. She’s not afraid of him — more than you can say for Collins.

In the most important Senate race in the country next year, Maine Democrats have a responsibility to take their best shot.

Party leaders in Washington appreciate that when the stakes are high, you go with your best. When will Maine’s rank-and-file Democrats reach the same conclusion?